Osteoporosis affects more than 28 million Americans and accounts for more than 1.5 million fractures annually, at a cost of 513.8 billion. One of the most effective ways to prevent osteoporosis is to build a strong, dense skeleton during the growing years to act as a reserve in later life. Furthermore, changes in individual behaviors and lifestyle preferences in younger children may last through the critical adolescent period and continue into adulthood, providing a lifetime of optimal bone health. The primary aim of this study is to develop, implement and evaluate a multi-faceted intervention in after-school programs that will increase bone accretion and muscular strength in early elementary school children. The intervention will include weight loading physical activity and calcium-rich snacks for children as well as age- appropriate, behaviorally focused active learning to promote skill building and self-competence. It will also include a parent/caregiver communications module that by provides specific activities and practical information to complement and reinforce the direct intervention with the children. A mascot and logo will be used to foster positive attitudes about calcium-rich foods and physical activity; incentives and goal setting will be used to encourage sustained behavior. One thousand eight hundred first and second grade children from 60 after-school programs in up to 10 diverse communities throughout Massachusetts will be randomized and studied for 24 months. Matched program groups will receive either The BONES Project (n=24), The BONES Project plus a parent/caregiver component (n=12); or they will serve as controls (n=24). Follow-up will continue for 12 months after the intervention period. The primary outcomes will be bone quality and muscular strength. Secondary outcomes will include physical activity levels, calcium consumption, body composition, self-perception of physical abilities and appearance, and influencing factors toward relevant healthy lifestyle behaviors, such as knowledge and attitudes. The control group will receive a "delayed" intervention after termination of the 36-month study. Focus group research with after-school program leaders, parents/caregivers, and children prior to initiation will provide information to optimize the campaign design. Findings from this study will promote our understanding of health behaviors in young children and the effect of parental involvement on these behaviors. Implementation of the same intervention within this widely diverse set of communities will guide appropriate cultural, community and socioeconomic-specific modifications and permit easier replication of the program. Because the intervention utilizes the existing after-school framework large numbers of children can be reached. Results from this study will provide an appropriate model for national replication and sustainability of a program to beat osteoporosis by nourishing and exercising skeletons early in life.